U.S. patent number 6,894,644 [Application Number 10/483,485] was granted by the patent office on 2005-05-17 for radio positioning systems.
This patent grant is currently assigned to Cambridge Positioning Systems Limited. Invention is credited to John Christopher Clarke, Peter James Duffett-Smith, Malcolm David MacNaughtan, Robert Willem Rowe.
United States Patent |
6,894,644 |
Duffett-Smith , et
al. |
May 17, 2005 |
Radio positioning systems
Abstract
The present invention sets out to overcome the hearability
problem in CDMA communications networks in which positioning
services are provided, by using a separate sampling device (204,
205, 206) for each transmitter (201, 202, 203), which sends to the
computing device (208) a representation of the signals transmitted
only by that transmitter. A cross-correlation of the representation
sent back by the mobile terminal (207) with the representation sent
back by the sampling device in the brightest transmitter is
performed in the computing device (208), and an estimate of that
brightest signal is subtracted from the representation sent back by
the mobile terminal (207) in order to reduce its effect on the
remaining signals as far as possible. The cross-correlation and
subtraction steps are iterated until no useful signals remain to be
extracted.
Inventors: |
Duffett-Smith; Peter James
(Cambridge, GB), MacNaughtan; Malcolm David (Sydney,
AU), Clarke; John Christopher (Cambridge,
GB), Rowe; Robert Willem (Cambridge, GB) |
Assignee: |
Cambridge Positioning Systems
Limited (Cambridge, GB)
|
Family
ID: |
8182113 |
Appl.
No.: |
10/483,485 |
Filed: |
January 13, 2004 |
PCT
Filed: |
July 15, 2002 |
PCT No.: |
PCT/GB02/03253 |
371(c)(1),(2),(4) Date: |
January 13, 2004 |
PCT
Pub. No.: |
WO03/00899 |
PCT
Pub. Date: |
January 30, 2003 |
Foreign Application Priority Data
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|
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|
|
Jul 17, 2001 [EP] |
|
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01306115 |
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Current U.S.
Class: |
342/387; 342/450;
342/464 |
Current CPC
Class: |
G01S
5/0215 (20130101); G01S 5/02 (20130101) |
Current International
Class: |
G01S
5/10 (20060101); G01S 1/00 (20060101); G01S
1/04 (20060101); G01S 5/02 (20060101); G01S
001/24 (); G01S 003/02 () |
Field of
Search: |
;342/387,450,464 |
References Cited
[Referenced By]
U.S. Patent Documents
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5838279 |
November 1998 |
Duffett-Smith et al. |
6094168 |
July 2000 |
Duffett-Smith et al. |
6275705 |
August 2001 |
Drane et al. |
6342854 |
January 2002 |
Duffett-Smith et al. |
6522890 |
February 2003 |
Drane et al. |
6529165 |
March 2003 |
Duffett-Smith et al. |
2002/0149518 |
October 2002 |
Haataja et al. |
|
Foreign Patent Documents
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1118871 |
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Jul 2001 |
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EP |
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1185878 |
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Mar 2002 |
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EP |
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1235076 |
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Aug 2002 |
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EP |
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1255122 |
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Nov 2002 |
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EP |
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1271178 |
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Jan 2003 |
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EP |
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1278074 |
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Jan 2003 |
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EP |
|
1185877 |
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Mar 2003 |
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EP |
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1301054 |
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Apr 2003 |
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EP |
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9730360 |
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Aug 1997 |
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WO |
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9911086 |
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Mar 1999 |
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WO |
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WO 99/21028 |
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Apr 1999 |
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WO |
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Other References
Lopes, L. et al, "GSM Standards Activity on Location," 1999
IEEE..
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Primary Examiner: Issing; Gregory C.
Attorney, Agent or Firm: Roylance, Abrams, Berdo &
Goodman, L.L.P.
Claims
We claim:
1. A method of finding the position or state of motion of a
terminal in a communications network having a plurality of
transmitters, the terminal having a radio receiver, the method
comprising the steps of (a) creating at the terminal a section of a
representation of the signals from the transmitters received by the
radio receiver; (b) creating a first section of a representation of
the signal transmitted by a first of said transmitters, and
creating a second section of a representation of the signals
transmitted by a second of said transmitters, each of which
sections overlaps in time with the section created at the terminal;
(c) calculating an estimate of the signal received at the terminal
from said first transmitter using said first section, and
subtracting said estimate from the section created at the terminal,
to produce a residual representation; (d) performing a calculation
using said residual representation and said second section, and
estimating the time offset between them; and (e) calculating the
position of the terminal using said time offset.
2. A method of finding the position or state of motion of a
terminal in a communications network having a plurality of
transmitters, the terminal having a radio receiver, the method
comprising the steps of (a) creating at the terminal a section of a
representation of the signals from the transmitters received by the
radio receiver; (b) creating a section of a representation of the
signal transmitted by one of said transmitters, which overlaps in
time with the section created at the terminal; (c) calculating an
estimate of the signal received at the terminal from said
transmitter using said section of the representation of the signal
transmitted by said transmitter, and subtracting said estimate from
the section created at the terminal, to produce a residual
representation; (d) performing one or more calculations using said
residual representation and one or more known components of the
signals transmitted by the communications network in order to
estimate the time offset of the respective component; and (e)
calculating the position of the terminal using any of said time
offsets.
3. A method of finding the time offset between at least one of the
signals received from a plurality of the transmitters of a
communications network by a receiver attached to a fixed terminal
and a reference generated in the fixed terminal, the method
comprising the steps of (a) creating at the fixed terminal a
section of a representation of the signals from the transmitters
received by the radio receiver; (b) creating a section of a
representation of the signal transmitted by one of said
transmitters, which section overlaps in time with the section
created at the fixed terminal; (c) calculating an estimate of the
signal received at the fixed terminal from said transmitter using
said section of a representation of the signal transmitted by said
transmitter, and subtracting said estimate from the section created
at the fixed terminal, to produce a residual representation; and
(d) performing a calculation using said residual representation and
said reference in order to estimate the time offset between the at
least one signal and said reference.
4. A method according to any of claims 1, 2 or 3, wherein the or
each section of a representation of the signal transmitted by a
respective transmitter is created at a respective transmitter.
5. A method according to any of claims 1, 2 or 3, wherein the or
each section of a representation of the signal transmitted by a
respective transmitter is created in a sampling device associated
with the respective transmitter.
6. A method according to any of claims 1, 2 or 3, in which the
signal representation sections are sent to one or more computing
devices in which said estimates and time offsets are
calculated.
7. A method according to claim 6, in which the terminal location is
calculated in said one or more computing devices.
8. A method according to claim 1, in which the time offset between
said section of a representation of the signals received by the
receiver and said first section is first calculated, and then used
in the calculation of said estimate.
9. A method according to any of claims 1, 2 or 3, in which the
section of the representation of the signals received by the
receiver at the terminal is recorded in the terminal before being
sent to one or more computing devices.
10. A method according to any of claims 1, 2 or 3, in which the
section of the representation of the signals received by the
receiver at the terminal is transferred in real time to one or more
computing devices and a recording or recordings made there.
11. A method according to claim 6, in which the computing device is
in the handset.
12. A method according to claim 6, in which one or more of the
computing devices each comprise a processor connected to the
network.
13. A method according to any of claims 1, 2 or 3, in which the
representation of the signals received by the receiver is a
digitised version of the received signals converted first to
baseband in the receiver.
14. A method according to any of claims 1, 2 or 3, in which the
representation of the signals transmitted by a transmitter is a
digitised version of the transmitted signals converted first to
baseband.
15. A method according to any of claims 1, 2 or 3, in which, in
order to ensure an overlap of the respective sections, a known
component of the transmitted signals is used to indicate the start
of sampling.
16. A method according to any of claims 1, 2 or 3, in which the
calculations performed include cross-correlations.
17. A method according to claim 2, in which the known components of
the transmitted signals are the pilot codes.
18. A communications network comprising (a) a computing device or
devices; (b) a terminal having a radio receiver attached to the
terminal, means for creating a section of a representation of the
signals, received by the radio receiver, from the transmitters of
the communications network, and means for sending the section to
the computing device or devices; (c) sampling devices associated
with respective first and second of said transmitters for creating
respective first and second sections of representations of the
signal transmitted by the respective transmitter which overlap in
time with the section created at the terminal, and for sending the
sections of the representations created at said transmitters to
said computing device or devices; the computing device or devices
being adapted to perform 1. a calculation of an estimate of the
signals received at the terminal from said first transmitter using
said first section; 2. a subtraction of said estimate from the
section sent by the terminal, to produce a residual representation;
3. a calculation using said residual representation and said second
section to produce an estimate of the time offset between them; and
4. a calculation of the position of the terminal using said time
offset.
19. A communications network comprising (a) a computing device or
devices; (b) a terminal having a radio receiver attached to the
terminal, means for creating a section of a representation of the
signals, received by the radio receiver, from the transmitters of
the communications network, and means for sending the section to a
computing device; (c) devices associated with the transmitters for
creating sections of representations, of the signal transmitted by
the respective transmitter, which overlap in time with the section
created at the terminal, and for sending said sections to the
computing device or devices; the computing device or devices being
adapted to perform 1. generation of a reference signal; 2.
calculation of an estimate of the signal received at the terminal
from said transmitter using said section of a representation of the
signal transmitted by the corresponding transmitter; 3. a
subtraction of said estimate from the section sent by the terminal
to produce a residual representation; 4. one or more calculations
using said residual representation and said reference to estimate
the time offset between a component of the residual representation
and said reference; and 5. a calculation of the position of the
terminal using the or any of the time offsets.
20. A communications network according to claim 18 or claim 19, in
which the, section of the representation of the signals received by
the receiver at the terminal is recorded in the terminal before
being sent to a computing device.
21. A communications network according to claim 18 or claim 19, in
which the section of the representation of the signals received by
the receiver at the terminal is transferred in real time to a
computing device and a recording made there.
22. A communications network according to claim 18 or claim 19, in
which the section of the representation of the signal transmitted
by a transmitter is obtained from a sampling device associated with
the corresponding transmitter.
23. A communications network according to claim 18 or claim 19, in
which the computing device is in a handset.
24. A communications network according to claim 18 or claim 19, in
which the computing device comprises a processor connected to the
network.
25. A communications network according to claim 18 or claim 19, in
which the representation received by the receiver is a digitised
version of the received signals converted first to baseband in the
receiver.
26. A communications network according to claim 18 or claim 19, in
which the representation of the signal transmitted by a transmitter
is a digitised version of the transmitted signal converted first to
baseband.
27. A communications network according to claim 18 or claim 19, in
which, in order to ensure an overlap of the respective sections, a
known component of the transmitted signals is used to indicate the
start of sampling.
28. A communications network according to claim 18 or claim 19, in
which the calculations performed in the computing device include
cross-correlations.
29. A communications network according to claim 19, in which the
component of the residual representation is a pilot code.
30. A computing device or devices for use in a communications
network comprising a terminal having a radio receiver attached to
the terminal, means for creating a section of a representation of
the signals, received by the radio receiver, from the transmitters
of the communications network, and means for sending the section to
the computing device or devices; and sampling devices associated
with respective first and second of said transmitters for creating
respective first and second sections of representations of the
signal transmitted by the respective transmitter which overlap in
time with the section created at the terminal, and for sending the
sections of the representations created at said transmitters to
said computing device or devices; the computing device or devices
being adapted to perform 1. a calculation of an estimate of the
signals received at the terminal from said first transmitter using
said first section; 2. a subtraction of said estimate from the
section sent by the terminal, to produce a residual representation;
3. a calculation using said residual representation and said second
section to produce an estimate of the time offset between them; and
4. a calculation of the position of the terminal using said time
offset.
31. A computing device or devices for use in a communications
network comprising a terminal having a radio receiver attached to
the terminal, means for creating a section of a representation of
the signals, received by the radio receiver, from the transmitters
of the communications network, and means for sending the section to
the computing device or devices; and devices associated with the
transmitters for creating sections of representations, of the
signal transmitted by the respective transmitter, which overlap in
time with the section created at the terminal, and for sending said
sections to the computing device or devices; the computing device
or devices being adapted to perform 1. a generation of a reference
signal; 2. a calculation of an estimate of the signal received at
the terminal from said transmitter using said section of a
representation of the signal transmitted by the corresponding
transmitter; 3. a subtraction of said estimate from the section
sent by the terminal to produce a residual representation; 4. one
or more calculations using said residual representation and said
reference to estimate the time offset between a component of the
residual representation and said reference; and 5. a calculation of
the position of the terminal using the or any of the time
offsets.
32. A computer program or programs comprising computer program code
means adapted to perform the steps of the computing device of claim
30.
33. A computer program or programs comprising computer program code
means adapted to perform the steps of the computing device of claim
31.
Description
The present invention relates to radio positioning systems
generally, and more particularly to improved methods of finding the
positions of mobile terminals in radio communication systems,
especially those employing Code Division Multiple Access (CDMA)
technology.
There are many systems known by which the position of a mobile
terminal operating in a radio communications network may be
determined. These include using the signals from transmitters not
connected with the network, such as the Global Positioning System
(GPS) satellites, but others make use of the signals radiated by
the mobile terminal and picked up by remote receivers, such as the
Time Of Arrival (TOA) and so-called "Radio Finger Printing" systems
or, vice versa, using the signals radiated by the network itself
and picked up by the mobile terminal. Chief amongst the last
category are the Enhanced Observed Time Difference (E-OTD) and
Observed Time Difference Of Arrival (OTDOA) systems.
The E-OTD system, although generally applicable to many different
communication technologies, has been particularly applied to the
Global System for Mobiles (GSM). Two principal, and different,
methods of using the timing offsets of signals received from the
network transmitters in the position computation have been
described in the art. In one, e.g. EP-A-0767594, WO-A-9730360 and
AU-B-716647, the signals measured by a fixed receiver are used, in
effect, to `synchronise` the transmissions from the different
transmitters. The instantaneous transmission time offsets of each
transmitter relative to its neighbours are calculated from the
values measured at the fixed receiver using the known positions of
the fixed receiver and the transmitters. The timing offsets
measured by the mobile terminal can then be used in a calculation
based on well-known standard techniques in which the points of
intersection of two or more hyperbolic position lines predict the
position of the mobile terminal.
The other method (see our EP-B-0303371, U.S. Pat. No. 6,094,168 and
EP-A-1025453 the details of which are hereby incorporated by
reference and which refer to a system known as Cursor.TM.) makes
use of the measurements made by both the fixed receiver and the
mobile terminal to calculate the relative time difference between
the signals received from each transmitter by both receivers. This
results in a calculation based on the intersection of circles
centred on the transmitters.
E-OTD methods, as applied to GSM, have been considered for use in
wide-band CDMA systems, in particular those within the Universal
Mobile Telephone System (UMTS) `third generation` (3G)
technologies. Here, E-OTD has been re-named OTDOA, but it suffers
from a major problem, the so-called `hearability` problem. In CDMA
networks generally, signals are transmitted by the network
transmitters all using the same radio frequency (RF) channel. In
UMTS this channel is about 5 MHz wide. The signals from each
transmitter are encoded using a unique `spreading code` which
allows a mobile terminal to pick out the required signal provided
that (a) it knows the spreading code used by that transmitter, and
(b) its internal clock is synchronized with the transmitter
signals. To assist with the latter, each transmitter also radiates
a `pilot code` within the same RF channel whose coding and other
characteristics make it easily distinguishable. The mobile terminal
first detects and locks on to the pilot signal, receives the
spreading code used by that transmitter, and then is able to decode
the main transmissions. The hearability problem arises when the
mobile terminal is near to a transmitter. E-OTD systems (and
therefore OTDOA systems) require the measurements of the time
offsets associated with at least three geographically-distinct
transmitters, but when the mobile terminal is too close to a
transmitter, the signals from the more-distant transmitters are
drowned out by the local signals to the extent that their time
offsets cannot be measured. One technique, known as `Idle Period
Down Link` (IPDL), has been proposed to overcome this problem by
which the transmissions from the local transmitter are turned off
periodically in a so-called `idle period` during which the signals
from the distant transmitters may be received. This has the serious
disadvantages that (a) the capacity of the network to carry voice
& data traffic is diminished, and (b) it is complicated to
install and operate, requiring in one of its forms additional
messaging in the network to coordinate the idle periods amongst the
transmitters.
The present invention involves an adaptation of the Cursor.TM.
system, especially as described in our U.S. Pat. No. 6,094,168, to
CDMA systems in general and particularly to UMTS in such a fashion
as to overcome the hearability problem. No idle period is required,
and the communications function can therefore operate with full
capacity. It has the further advantages that (a) the fixed
receivers associated with E-OTD and OTDOA are particularly simple
and low-cost devices, and (b) the additional software required in
the mobile terminals is less complex than that in GSM
terminals.
The Cursor.TM. system, as described in U.S. Pat. No. 6,094,168,
uses two receivers, one fixed and at a known location and the other
within the mobile terminal, to receive the signals radiated by each
transmitter taken separately. Representations of the received
signals are sent back to a computing node where they are compared
(generally by cross-correlation) to determine the time offset of
receipt of the signals by each receiver. This process is repeated
for at least two other geographically distinct transmitters
(transmitting on different RF channels in a GSM system) to obtain
the three time offsets required for a successful position
computation.
In direct sequence CDMA systems the transmitters use the same RF
channel. A direct application of the Cursor.TM. system to CDMA
would therefore result in a cross-correlation with many peaks, each
corresponding to the alignment of the signals received from a
particular one of the transmitters by both receivers. If it were
possible to measure the peaks associated with at least the three
required transmitters, the system would serve admirably for
positioning. However, as illustrated in the particular embodiment
described below, the signal to noise ratios (SNRs) associated with
more-distant transmitters are often too small, and we have a
similar hearability problem as described above.
A first aspect of the invention therefore provides a method of
finding the position or state of motion of a terminal in a
communications network having a plurality of transmitters, the
terminal having a radio receiver, the method comprising the steps
of (a) creating at the terminal a section of a representation of
the signals from the transmitters received by the radio receiver;
(b) creating a first section of a representation of the signal
transmitted by a first of said transmitters, and creating a second
section of a representation of the signals transmitted by a second
of said transmitters, each of which sections overlaps in time with
the section created at the terminal; (c) calculating an estimate of
the signal received at the terminal from said first transmitter
using said first section, and subtracting said estimate from the
section created at the terminal, to produce a residual
representation; (d) performing a calculation using said residual
representation and said second section, and estimating the time
offset between them; and (e) calculating the position of the
terminal using said time offset.
Preferably, the first and second sections are created at the
respective first and second transmitters, but they may be created
elsewhere. They may be created in one or more sampling devices
attached to the respective transmitters or located elsewhere, or
they may be created by computer programs running anywhere in the
communications network, or elsewhere, using information supplied
from the network about the transmitted signals.
The various signal representation sections may be sent to one or
more computing devices in which said estimates and time offsets,
and the terminal location, may be calculated. In some embodiments,
the time offset between said section of a representation of the
signals received by the receiver and said first section may first
be calculated, and may then be used in the calculation of said
estimate. The time offset may be calculated using said sections or
it may be calculated by other means, for example by calculating the
time offset of a known component of the signal such as the pilot
code.
The present invention thus overcomes the hearability problem by,
for example, using a separate sampling device for each transmitter,
the equivalent of the fixed receiver in an E-OTD system, which
sends to a computing device a representation of the signals
transmitted only by that transmitter, by performing a
cross-correlation of the representation sent back by the mobile
terminal with the representation sent back by the sampling device
associated with one of the transmitters to estimate the time offset
between them, and by subtracting an estimate of that signal from
the representation sent back by the mobile terminal in order to
reduce its effect on the remaining signals as far as possible. The
cross-correlation and subtraction steps may be iterated until no
useful signals remain to be extracted. Simulations show that this
provides at least as much hearability gain as the IPDL method.
However, of course, the transmitted signals are unaffected by the
method of the invention, so that, for example, the transmissions do
not need to be interrupted.
In some systems, the hearability problem may be solved simply by
subtracting an estimate of just one of the signals, usually the
brightest, leaving a residual representation in which the time
offsets of the pilot codes, or any other known portions of the
transmitted signals, may be determined.
Thus, a second aspect of the invention provides a method of finding
the position or state of motion of a terminal in a communications
network having a plurality of transmitters, the terminal having a
radio receiver, the method comprising the steps of (a) creating at
the terminal a section of a representation of the signals from the
transmitters received by the radio receiver; (b) creating a section
of a representation of the signal transmitted by one of said
transmitters, which overlaps in time with the section created at
the terminal; (c) calculating an estimate of the signal received at
the terminal from said transmitter using said section of the
representation of the signal transmitted by said transmitter, and
subtracting said estimate from the section created at the terminal,
to produce a residual representation; (d) performing one or more
calculations using said residual representation and one or more
known components of the signals transmitted by the communications
network in order to estimate the time offset of the respective
component; and (e) calculating the position of the terminal using
any of said time offsets.
The section of the representation of the signals received by the
receiver at the terminal may be recorded in the terminal before
being sent to a computing device. Alternatively, the section may be
transferred in real time to the computing device and a recording
made there.
Preferably, the section of the representation of the signals
transmitted by a transmitter is created at said transmitter, but it
may be created elsewhere. It may be created in a sampling device
attached to said transmitter or located elsewhere, or it may be
created by a computer program running anywhere in the
communications network, or elsewhere, using information supplied
from the network about the transmitted signals.
The calculations may be carried out in a computing device which may
be in the handset or elsewhere, for example, a processor connected
to the network.
The representation of the signals received by the receiver attached
to the terminal may be a digitised version of the received signals
converted first to baseband in the receiver. The representation of
the signals transmitted by a transmitter may be a digitised version
of the transmitted signals converted first to baseband.
In order to ensure an overlap of the respective sections, a
suitably chosen component of the transmitted signals may be used to
indicate the start of sampling.
The time offset between said section of the signals received from a
transmitter by the receiver attached to the terminal and said
section of a representation of the signal transmitted by one of
said transmitters may be computed using a cross-correlation or
other comparison between the respective sections, or it may be
computed as part of the normal communications process in the
terminal, or it may be computed using a known component of the
signals transmitted by the communications network, for example a
pilot code.
The known components of the transmitted signals in the second
aspect of the invention may, for example, be the pilot codes.
The invention also includes apparatus for carrying out the
invention.
Thus, there is provided, for use in the carrying out the method of
the first aspect of the invention, a communications network, the
network comprising (a) a computing device or devices; (b) a
terminal having a radio receiver attached to the terminal, means
for creating a section of a representation of the signals, received
by the radio receiver, from the transmitters of the communications
network, and means for sending the section to the computing device
or devices; (c) sampling devices associated with respective first
and second of said transmitters for creating respective first and
second sections of representations of the signal transmitted by the
respective transmitter which overlap in time with the section
created at the terminal, and for sending the sections of the
representations created at said transmitters to said computing
device or devices; the computing device or devices being adapted to
perform 1. a calculation of an estimate of the signals received at
the terminal from said first transmitter using said first section;
2. a subtraction of said estimate from the section sent by the
terminal, to produce a residual representation; 2. a calculation
using said residual representation and said second section to
produce an estimate of the time offset between them; and 3. a
calculation of the position of the terminal using said time
offset.
The invention also includes a computing device or devices for use
in such a communications network, adapted to perform the tasks set
out in the paragraph immediately above.
The invention also includes, for use in the carrying out the method
of the second aspect of the invention, a communications network
comprising (a) a computing device or devices; (b) a terminal having
a radio receiver attached to the terminal, means for creating a
section of a representation of the signals, received by the radio
receiver, from the transmitters of the communications network, and
means for sending the section to a computing device; (c) devices
associated with the transmitters for creating sections of
representations, of the signal transmitted by the respective
transmitter, which overlap in time with the section created at the
terminal, and for sending said sections to the computing device or
devices;
the computing device or devices being adapted to perform 1. a
generation of a reference signal; 2. a calculation of an estimate
of the signal received at the terminal from said transmitter using
said section of a representation of the signal transmitted by the
corresponding transmitter; 3. a subtraction of said estimate from
the section sent by the terminal to produce a residual
representation; 4. one or more calculations using said residual
representation and said reference to estimate the time offset
between the at least one signal and said reference; and 5. a
calculation of the position of the terminal using the or any of the
time offsets.
The invention also includes a computing device or devices for use
in such a communications network, adapted to perform the tasks set
out in the paragraph immediately above.
The means for carrying out the calculations in the computing device
or devices may be components of hardware and/or software.
Therefore, the invention includes a computer program or programs
having computer program code means for carrying out the steps
performed in the computing device or devices as described
above.
The terminal may be a part of a positioning system, for example as
described in any of EP-A-0767594, WO-A-9730360, AU-B-716647
EP-B-0303371, U.S. Pat. No. 6,094,168 and EP-A-1025453 and may be a
fixed device associated with a transmitter (for example, the `fixed
receiver` or `Location Measurement Unit, LMU`), whose purpose is to
receive signals from distant transmitters as well as from its
associated transmitter, in which case the method of the invention
includes the estimation of and subtraction of the signals from its
associated transmitter in order to allow it to measure the time
offsets of the signals received from distant transmitters.
A third aspect of the invention therefore provides a method of
finding the time offset between at least one of the signals
received from a plurality of the transmitters of a communications
network by a receiver attached to a fixed terminal and a reference
generated in the fixed terminal, the method comprising the steps of
a) creating at the fixed terminal a section of a representation of
the signals from the transmitters received by the radio receiver;
b) creating a section of a representation of the signal transmitted
by one of said transmitters, which section overlaps in time with
the section created at the fixed terminal; c) calculating an
estimate of the signal received at the fixed terminal from said
transmitter using said section of a representation of the signal
transmitted by said transmitter, and subtracting said estimate from
the section created at the fixed terminal, to produce a residual
representation; d) performing a calculation using said residual
representation and said reference in order to estimate the time
offset between a component of the residual representation and said
reference.
The E-OTD positioning systems described generally above work with
unsynchronised networks, i.e. any common component of the signals
transmitted by any one transmitter is not synchronised in time with
the transmission of that component by any other of the
transmitters, but instead is transmitted after an unknown time
delay, sometimes called the Relative Transmission Delay (RTD). The
position calculation requires that this delay is known, and so the
positioning systems employ fixed receivers at known locations
throughout the network which are set up to measure the transmitted
signals and compute the RTDs. It has been described above how the
hearability problem hinders the straightforward application of the
E-OTD techniques to direct-sequence CDMA systems. However, the
third aspect of the present invention overcomes the hearability
problem by allowing the very strong signals from a local
transmitter to be subtracted from the signals received by the fixed
receiver, thus allowing the much weaker signals from the distant
transmitters to be measured. The method of application of E-OTD to
CDMA systems then follows that described, for example, in our
EP-A-1025453.
The invention may be further understood by reference to the
accompanying drawings, in which:
FIG. 1 shows the geometry of a two-dimensional communications
system in which all the transmitters and the mobile terminal lie in
one plane;
FIG. 2 shows a simplified UMTS network;
FIG. 3 illustrates the correlation of a reference copy of the
primary scrambling code used on the pilot code channel (CPICH) by
each Node B of the UMTS network with a recording of the received
signal;
FIG. 4 shows the result of cross-correlating the recording received
by a terminal with recordings of the transmitted signals;
FIG. 5 illustrates measured and estimated recordings;
FIG. 6 show the cross-correlation of a residual recording with
recordings of transmitted signals; and
FIG. 7 shows the cross-correlation of a further residual recording
with a recording of a transmitted signal.
The following mathematical analysis provides an understanding of
the concepts involved in the present invention. FIG. 1 shows the
geometry of a two-dimensional system in which all the transmitters
and the mobile terminal lie in one plane. The positions of
transmitters A, B, and C are represented by the vectors a, b, c,
all with respect to the same common origin, O. The mobile terminal,
R, is at vector position r. Each of the transmitters has
incorporated with it a sampling device, as described above, which
samples the signals transmitted by that transmitter and which sends
back to a computing device (not shown in FIG. 1) a representation
thereof. Let us suppose that the mobile terminal is nearest to
transmitter A, then B, then C. The computing device first performs
a cross-correlation between the representation of the signals
received from A, B, and C (all on the same RF channel) by R, and
the representation of the signals transmitted by A. Since the
signals from A, B, and C have orthogonal spreading codes, the
cross-correlation results in a single peak whose position
represents the time-offset of the receipt of the signals from A by
R, together with the clock error, .epsilon., of the receiver in the
mobile terminal. This time offset, .DELTA.t.sub.A, is given by
where v is the speed of the radio waves, and the vertical bars
denote the magnitude of the contained vector quantity. Similarly,
for B and C we have
Having established the time offset of the signals from A, the
computing node now subtracts an estimate of the signal received
from A by R. The representations of the signals radiated at time t
by the transmitters A, B, and C, may be denoted by the functions
S.sub.A (t), S.sub.B (t), and S.sub.C (t) respectively. The signal
received by the mobile terminal comprises a combination of these.
In the absence of multipath, noise and non-linear effects, the
representation of the received signals may be denoted by V(t),
where
and .alpha., .beta., .gamma.are constants representing the path
losses to the mobile terminal from the respective transmitters. A
software program running in the computing node estimates the
magnitude of S.sub.A (t), delayed by .DELTA.t.sub.A, to subtract
from V(t), for example by finding the value of a which minimises
the mean square amplitude of the residual V'(t). In the perfect
case this would remove the contribution of A altogether, so
that
The cross-correlation is now carried out between V'(t) and S.sub.B
(t) to estimate .DELTA.t.sub.B, and a further subtraction made to
remove the contribution of B from the residual, V"(t), where
if the subtraction is perfect. Finally, a cross-correlation between
V"(t) and S.sub.C (t) results in an estimate of .DELTA.t.sub.C.
Equations {1} can then be solved for r as described in U.S. Pat.
No. 6,094,168.
In practice, the signals received by the mobile terminal are
corrupted by noise, interference and multipath effects.
Furthermore, the representations of the signals may be in a digital
format of low resolution. The process of subtraction will not be
perfect in these circumstances, but may nevertheless be sufficient
to overcome the hearability problem. Where it is possible to
estimate the channel parameters, the effects of multipath
propagation can be allowed for, resulting in better signal
subtractions.
One of the requirements of the invention is that the recordings of
the signals made at A, B, C, and R overlap in time with each other.
The recording process in the mobile terminal can be initiated, for
example, by the receipt of a particular aspect of the signal
transmitted by the serving transmitter (A in the above analysis).
The recordings made in the transmitters must all be loosely
synchronised with this aspect. Where the transmitters are
synchronised with each other, as in the IS 95 standard, the aspect
will be transmitted at approximately the same time by all
transmitters in the network. In unsynchronised systems, however,
other means such as GPS or the concepts described in our
WO-A-00/73814 and EP application no. 01301679.5 may be used for
synchronisation.
One embodiment of a system according to the invention will now be
described with reference to FIGS. 2 to 7.
FIG. 2 shows a simplified UMTS system consisting of three
communications transmitters (Node Bs) 201, 202, 203, each of which
has a sampling device 204, 205, 206, a single terminal (user
equipment, UE) 207, and a computing device (serving mobile location
centre, SMLC) 208. Each Node B has an omnidirectional antenna, and
is configured to transmit signals typical of network traffic load.
Table 1 below indicates the different physical channels in use,
together with their power levels and symbol rates. The acronyms
appearing in the left-hand column, P-CPICH etc., are those that
have been adopted by the industry to represent the channels. Random
binary sequences are used to modulate the DPCHs. The three Node Bs
use orthogonal primary scrambling codes, in this case numbers 0, 16
and 32 respectively.
TABLE 1 Node B channel configuration Channel Relative power
Level/dB Symbol rate/Kss.sup.-1 P-CPICH -10 15 P-SCH -10 15 S-SCH
-10 15 P-CCPCH -10 15 PICH -15 15 DPCH0 Note 1 Note 2 DPCH1 Note 1
Note 2 DPCH2 Note 1 Note 2 . . . Note 1 Note 2 . . . Note 1 Note 2
DPCH63 Note 1 Note 2 DPCH64 Note 1 Note 2 Note 1: DPCH power levels
were chosen randomly from -10 dB to -25 dB Note 2: DPCH symbol
rates were chosen randomly from 15 to 240 Kss.sup.-1
The Node Bs are tightly synchronised. This is not a requirement in
normal practice, but is convenient for the purpose of
demonstration.
It will be noted from FIG. 2 that the UE 207 is relatively close to
Node B 201 and at greater distances from Node Bs 202 and 203. Thus
the signal from Node B 201 is the strongest (0 dB relative to
itself) with the signal from Node B 202 weaker at -15 dB and that
from Node B 203 weakest of all at -30 dB. The three sampling
devices 204, 205, 206 are instructed by the SMLC 208 to record and
report the signal transmitted by the associated Node B during the
first 256 chips immediately following the start of the next second.
These signals are sampled at a rate of 2 samples per chip, with a
resolution of 4 bits.
Before describing how the system is used to illustrate the present
invention, the problem of hearability is highlighted by considering
the conventional E-OTD approach to measuring the time offsets of
the signals received by the UE 207. A reference copy of the primary
scrambling code used on the CPICH by each Node B (i.e. the first
256 chips of each of scrambling codes 0, 16 and 32), is
cross-correlated with the signal received by the UE 207 and a
search is made for the highest correlation peak. FIG. 3 illustrates
a typical result. Note that the signals received by the UE 207 are
also sampled at a rate of 2 samples per chip, with a resolution of
4 bits. The resulting cross-correlation profiles show one clearly
distinguishable peak 301 in the correlation for scrambling code 0,
corresponding to the time offset of the signals from Node B 201.
However, the cross-correlation results for the codes 16 and 32 do
not yield any clear peaks. This is because the signals received by
the UE 207 from Node Bs 202 and 203 are swamped by the relatively
strong reception from Node B 201. Were they visible, these peaks
should be positioned to the right of the visible peak 301 by 1 and
2 microseconds respectively for the signals from Node Bs 202 and
203 (corresponding to 3.8 and 7.6 chips).
The lack of detection of the signals from 202 and 203 means that it
is not possible to compute an E-OTD position fix, since at least
three independent timings are needed. It has already been described
how the use of idle periods (e.g. the IPDL method) can be used to
overcome this problem.
The present invention is now illustrated using the same test
system. In this case, each sampling device 204, 205, 206 records a
section of the signals transmitted by its associated Node B 201,
202, 203 respectively. This section is one symbol in duration and
is again sampled at a rate of 2 samples per chip, with a resolution
of 4 bits. The UE 207 also records a 256-chip section of the
signals it receives, aligned with the first symbol on the CPICH in
a particular timeslot, at the same sampling rate and
resolution.
At the SMLC 208, the three recordings reported by the three
sampling devices 204, 205, 206 are each cross-correlated in turn
with the recording made by the UE 207, and the results are shown in
FIG. 4. The peaks of the resulting correlation profiles are used to
determine the relative levels of the three contributions in the
received signal and hence the order in which they are to be
subtracted. Once again, the cross-correlation for Node B 201 yields
the largest peak 401. Note also that, in contrast with FIG. 3, the
cross-correlation for Node B 202 also yields a clear peak 402. This
is because the cross-correlation is performed using the total
signal transmitted by the Node Bs rather than merely using the
CPICH, which represents a fraction of the total transmitted energy
in each case.
Having identified the time offset of the signal from Node B 201,
the recording of the signal reported by the sampling device 204 is
now used to construct an appropriately scaled, delayed and
phase-rotated copy of that signal. The results of this process are
plotted in FIG. 5. The upper plot shows the real component of the
original signal recorded by the UE 207 as a solid curve whilst the
dotted curve shows the estimated scaled, delayed and rotated
signal. The lower plot shows a similar comparison of the imaginary
parts of received and estimated signals. Note that whilst a
duration of 256 chips is actually used in the example, the time
axis in this Figure has been limited to about 50 chips. The
estimated recordings are subtracted from the total UE recording
leaving a residual recording.
The recordings from the sampling devices 205 and 206 are now
cross-correlated with the residual recording giving the results
shown in FIG. 6. Note that in this case, following the removal of
the signal from Node B 201, there is a clear correlation peak 601
for the signals from Node B 203 as well as a peak 602 for Node B
202. These peaks are used to estimate the time offsets of the
corresponding signals, giving sufficient independent timing
measurements (three in this case) to compute a position fix.
If the peak 601 corresponding to the signals from Node B 203 is too
weak to be resolved, a further iteration could be undertaken in
which the signals from Node B 202 could be subtracted to yield a
second residual signal (FIG. 7). There is a clear correlation peak
701 at a delay of approximately 7 chips as expected.
In summary therefore, the problem caused by the relatively high
level of the signals from Node B 201, which prevents measurement of
the time offsets for the signals from Node Bs 202 and 203 by the
conventional method, is overcome by the iterative approach of the
present invention which involves the estimation and subtraction of
the strongest remaining signal so that the next weaker one can be
detected.
As explained earlier, the present invention can also be applied to
the fixed receivers (LMUs) of a positioning system using
conventional E-OTD techniques. In this case, a fixed receiver is
usually co-sited with the transmitter, but is connected to a
separate receiving antenna. The LMU, which needs to support a large
dynamic range and display exceptionally good linear
characteristics, receives the signals picked up by its antenna,
creates a section of a representation of the signals as described
above, and sends the section to a computing device. A sampling
device associated with the transmitter provides a contemporaneous
section of the signal transmitted by the transmitter. A calculation
is then carried out in which an estimate of the signal transmitted
by the transmitter and picked up by the receiving antenna is
subtracted from the representation (to produce a residual
representation) in order to reduce its effect on the signals
received from the other, more-distant, transmitters of the network.
Thus far, the process is exactly as described in the particular
example discussed above. The purpose of the LMU, however, is to
furnish as many timing offsets between the signals received from
all the transmitters as it can. The very strong signal received
from the local transmitter furnishes an accurate timing for that
transmitter through analysis of the section of the representation
sent by the LMU to the computing device, and the residual
representation may then be analysed for the remaining signals from
other transmitters.
The analysis of both the representation and the residual
representation to find the time offset of a particular component
may be carried out as follows. The pilot code, transmitted on CPICH
by a given transmitter, is known in advance as a binary sequence.
This is modulated by passing it through, for example, a
raised-cosine filter so that it matches as closely as possible the
signal received from the transmitter on CPICH. This reference
sequence is then cross-correlated with the section of the
representation of the received signals, or the section of the
residual representation, in order to identify a peak corresponding
to the time offset of the signal received from the corresponding
transmitter with respect to the reference, as illustrated above in
FIG. 3.
* * * * *